Heredity 87 (2001) 257±265 Received 14 February 2001, accepted 26 June 2001

SHORT REVIEW

When does matter?

WILLIAM AMOS* & ANDREW BALMFORD Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, U.K.

Is this short review we explore the genetic threats facing retain more useful genetic variability than they `should', for declining populations, focusing in particular on empirical example by enhanced reproductive success among the most studies and the emerging questions they raise. At face value, outbred individuals in a population. Such ®ndings call into the two primary threats are slow erosion of genetic variability question the validity of simple models based on random by drift and short-term lowering of ®tness owing to inbreed- mating, and emphasize the need for more empirical data ing depression, of which the latter appears the more potent aimed at elucidating precisely what happens in natural force. However, the picture is not this simple. Populations populations. that have passed through a severe bottleneck can show a markedly reduced ability to respond to change, particularly Keywords: endangered species, evolutionary potential, gene- in the face of novel challenges. At the same time, several tic diversity, heterozygosity, inbreeding depression, population recent studies reveal subtle ways in which species are able to management

The perceived importance of genetic problems in the conser- or thousands of generations. Rate of loss is inversely vation of endangered species has ¯uctuated considerably over proportional to the genetic e€ective population size (Ne). In the last two decades, and remains the subject of debate. An practice, this means that the primary determinants of loss are early high-pro®le case study reported that cheetahs have low the size of the lowest population to date, and the time (in levels of genetic variability, poor sperm quality and poor generations) for which it has been held at around that level. reproductive success in captivity (O'Brien et al., 1983; O'Brien In contrast to loss of variability, the e€ects of inbreeding et al., 1985; O'Brien et al., 1986). It was concluded that the depression will begin to be felt within a few generations of a species had su€ered a genetic bottleneck, stripping it of decline, and their strength will depend primarily on the variability and leaving it prone to problems associated with magnitude of the drop in population size. This point is inbreeding depression. Although later studies (Caro & illustrated by considering a large population reduced to an Ne Laurenson, 1994; Caughley, 1994; Merola, 1994; Caro, 2000) of just 20. After ®ve generations at this size, essentially all have revealed many inconsistencies and the story has now been surviving individuals will be close relatives and, depending on largely rewritten, the cheetah project helped stimulate growing the breeding system, inbreeding depression would likely be at interest in the role of genetics in conservation. or near its most severe. In contrast, according to the well- Today, many conservation studies include a genetic element, known equation and the list of possible problems being considered has expanded to embrace loss of evolutionary potential, suscepti- t Ht ˆ H0 †1 1=2Ne ; bility to disease, mutational meltdown, and more. In this review we re-examine the main genetic threats faced by small where H is initial heterozygosity and H is heterozygosity t and declining populations and discuss the empirical basis for 0 t generations after a decline to size N (James, 1970), on average deciding which of these are most likely to pose serious e some 88% of heterozygosity at selectively neutral loci will problems over the sorts of time-scales that concern conserva- remain. tion practitioners. In doing so, we have made a conscious e€ort to look to the future by speculating about areas and concepts which are only now coming to light. Loss of variability Of the various possible genetic problems which face a When a population is driven to the brink of and declining population, loss of genetic variability and inbreed- held there for several generations, a large proportion of that ing depression have historically received most attention species' neutral variability may be eroded by chance loss of (O'Brien, 1994). Although often treated as one and the same alleles. One well-documented case of such comes phenomenon, these two processes are in reality very di€erent, from the Mauritius kestrel, which was reduced to a single pair and operate over radically di€erent timescales. In general, in the 1950s, recovered slowly at ®rst, but now numbers over variability is lost very slowly, usually over hundreds 500 birds. Comparisons of microsatellite diversity before and after this bottleneck reveal that some 50% of the heterozyg- *Correspondence. E-mail: [email protected] osity present in 19th century specimens has been lost, in good

Ó 2001 The Genetics Society of Great Britain. 257 258 W. AMOS & A. BALMFORD agreement with theoretical expectations based on the length 2000). Consequently, other factors must be responsible for the and depth of the bottleneck (Groombridge et al., 2000). species' current low nuclear variability. The possibility that Fortunately, the Mauritius kestrel looks to be the exception extreme but short-term depletion may have little impact on a not the rule. Using the above equation, it is easy to show that species' heterozygosity is supported by data from the Antarctic loss of heterozygosity is extremely slow compared with the fur seal. This species su€ered parallel and maybe more severe timescales over which conservation biology operates. For depletion at the hands of sealers (Bonner, 1968), yet currently example, a mammal with generation length of 10 years shows little evidence of (Wynen et al., 2000; reduced to an Ne or 50 in 1900 would still have 90% of its Gemmell et al., 2001). Results like these run counter to heterozygosity today. Species with shorter generation lengths intuition because moderate heterozygosity persisting in bottle- would experience greater loss per year for a given population necked populations may have been poorly reported (Amos & size, but such species generally exist at higher population Harwood, 1998). densities with larger e€ective population sizes even when in Two other points about heterozygosity are worth consider- decline. This expectation is borne out by a recent study in ing. The ®rst is that cheetahs and northern elephant seals are which expected losses of heterozygosity were calculated for 80 only two of many species which show very low variability for declining mammal populations, including most of the `classic' no clear reason. Several species have low heterozygosity examples of genetic depletion (Menchini et al. submitted). The despite no evidence of population decline, and carnivores in results are striking. Over 90% of all species are unlikely to particular appear to have low variability relative to other have lost more than 10% of their heterozygosity (see Fig. 1). mammals (Merola, 1994). One example is the European Thus, while many studies claim to show a link between badger, whose lack of variability for years frustrated biologists known population bottlenecks and low levels of genetic keen to use genetic markers to study its breeding behaviour variability (Hoelzel et al., 1993; O'Brien, 1994), closer exam- and population structure (accordingly this fact has not been ination usually reveals that the expected loss of heterozygosity published because it is a negative result!). Clearly, one should is far less than might be thought (Amos & Harwood, 1998). be cautious in inferring that low heterozygosity in any species, For example, claims that the cheetah has lost `90±99%' of its threatened or otherwise, is necessarily the result of bottlenec- variability through `one or more bottlenecks' (O'Brien, 1994) king (Amos & Harwood, 1998). are dicult to reconcile with a current population size The second point concerns the relationship between the numbering thousands (Amos & Harwood, 1998). Indeed, to variability being measured and the variability that is important lose 99% of its variability would require 16 generations of to the organism. Heterozygosity is only one measure of genetic sib±sib mating (Ne ˆ 2)! Similarly, the northern elephant seal is diversity, and tends to be less sensitive to population bottle- described as extremely genetically depleted (Hoelzel et al., necks than alternatives such as allelic diversity. More import- 1993), yet excellent historical records show that its bottleneck, antly, genetic diversity is usually assayed by (presumed) neutral although severe, probably only lasted two or three generations. markers. These re¯ect the passive loss of variability through Based on plausible parameter values, it seems highly unlikely genetic drift but are less informative about variability that that this decline could explain the loss of more than 25% of the impinges on ®tness. For example, a strongly balanced poly- species' nuclear genetic variability, although mitochondrial morphism like sickle cell anaemia in humans would be almost variability may have been a€ected more strongly (Weber et al., impossible to eliminate by bottlenecks alone. Consequently, loss of immediately useful variability will always tend to lag behind loss of neutral variability. Moreover, selection during population declines may favour heterozygosity itself through overdominance or genetic incompatibility (Tregenza & Wedell, 2000). For example during population crashes of Soay sheep, individuals that are heterozygous for adenosine deaminase show higher survival than homozygotes (Gulland et al., 1993), and it is noted that population collapses can actually trigger an increase in mean heterozygosity of the population as a whole (Bancroft et al., 1995; Pemberton et al., 1996).

The consequences of loss of variability Nevertheless, assuming that a species has lost useful variab- ility, what are the likely consequences? One possibility is that ®tness will be reduced as a direct consequence of a reduction in Fig. 1 Frequency distribution of the expected loss of genetic the number of heterozygous loci. This proposition stems from variability for 80 declining mammal populations, in relation to the fact that some, and maybe much, selectively important their conservation status (based on Hilton-Taylor, 2000). Loss variability is maintained by balanced polymorphisms in which of heterozygosity was estimated assuming the population the heterozygote is ®tter than either homozygote. Although declined instantaneously to its lowest recorded size, and has such locus-speci®c e€ects could operate entirely separately remained there since. From Menchini et al., submitted. from the more general aspects of inbreeding depression, in

Ó The Genetics Society of Great Britain, Heredity, 87, 257±265. CONSERVATION GENETICS OF ANNIHILATION 259 practice the two processes are virtually impossible to disentangle. Perhaps the main consequence of reduced variability is thought to lie in lowering a population's ability to react to novel challenges. Once again there is a huge discrepancy between the plausible and widespread assumption that adap- tability is compromised and the handful of empirical studies that provide direct evidence. Logically, the more important the genetic variability, both in terms of absolute selective value and the proportion of time any advantage is manifest (some variability may be neutral except under exceptional circum- stances), the more likely it would be to be retained. Hence, drift tends preferentially to remove the variability which is currently least important to the organism. This argument probably holds for challenges already being met by the organism, but may be less relevant for reaction to novelty. For example, Drosophila does not normally encounter high salinity, and when four populations of variously inbred and outbred ¯ies were exposed to increasing levels of salt (NaCl), the relatively outbred populations proved better able to adapt over time (Frankham et al., 1999; see Fig. 2). While this salinity challenge experiment provides support for a link between heterozygosity and evolutionary potential within a species, the relationship across species is less clear. Most obviously, the extreme polymorphism shown by mam- malian MHC genes is thought to be crucial for defence against disease, but evidence from cross-species comparisons is ambi- guous. On the one hand, Northern elephant seals show very low MHC diversity yet appear much less prone to a range of disease than Californian sealions which swim in the same waters and have not been bottlenecked (F. Gulland, pers. comm.). On the other hand, European harbour seals have very low diversity and su€ered very high mortality from Phocine Distemper Virus (PDV), which is endemic and less virulent in several arctic species. Whether the high mortality and rapid spread of PDV among harbour seals resulted from novel exposure, low MHC diversity or some combination is still Fig. 2 The ability of fruit ¯ies to tolerate increasing NaCl unclear, and illustrates the problems inherent in trying to concentrations, as a function of whether they are outbred interpret these complex and long-term causalities. Interest- (top), have been bottlenecked at one pair for one or three ingly, pathogens may not be the only factor maintaining high generations (1B, 3B), or have been highly inbred (bottom). The MHC diversity. Several vertebrates use MHC cues to detect, plot shows the NaCl concentrations at which populations and in some cases avoid mating with, relatives. Such beha- became extinct. Redrawn from Frankham et al. (1999). viours will tend to enhance MHC diversity regardless of any selective e€ects owing to disease. nomenon is known as inbreeding depression, and it can a€ect a population almost immediately following decline due to the Inbreeding depression overall increase in relatedness between individuals. The e€ect is At every generation, a species' genome su€ers many new transient because the reduced ®tness of relatively inbred , the majority of which are detrimental because there individuals purges the , the its strength is propor- are many more ways to disrupt gene function than there are to tional to the magnitude of the decline. improve it. In order to prevent genomic degeneration, these The consequences of inbreeding depression have been deleterious mutations must be removed as fast as they arise, widely reported and include reduced fecundity among pairs through di€erential mortality/fecundity. Because most genes in of close relatives and reduced survival of inbred individuals diploid organisms can operate satisfactorily with only a single (Ralls et al., 1970; Saccheri et al., 1996; Newman & Pilson, copy, loss of function mutations are generally recessive, 1997; Keller, 1998; Crnokrak & Ro€, 1999; see Hedrick & showing their e€ect only in the homozygous state. Conse- Kalinowski, 2000 for a recent review). For example, when the quently, loss of ®tness occurs when homozygosity is increased, butter¯y Bicyclus anynana is forced through a bottleneck of usually through matings between close relatives. This phe- one pair, fecundity drops to »20%, but often returns to normal

Ó The Genetics Society of Great Britain, Heredity, 87, 257±265. 260 W. AMOS & A. BALMFORD levels within a few generations of sustained inbreeding, breeding sites for small populations, many of which persist for presumably as a result of the purging of deleterious alleles only one or a few seasons (Saccheri et al., 1998). Here, inbred (Saccheri et al., 1996); see Fig. 3, and below). Parallel experi- populations have a higher extinction probability than outbred ments based on bottlenecks of ®ve pairs show a much reduced populations. Although it is early to generalize, these studies fecundity drop, and when 10 pairs are used, no detectable drop suggest that the most important e€ect of inbreeding depression is present (Saccheri et al., 1996). lies with its tendency to exacerbate the consequences of However, the picture is not straightforward. It is increas- environmental downturns. ingly evident that environmental stress such as food and water Until recently, inbreeding e€ects were studied almost shortage can uncover e€ects not seen under less demanding exclusively in families born to close relatives, either opportu- conditions. Convincing studies have been carried out in the nistically as in the Mandarte Island sparrows, or experiment- laboratory, for example using the ¯ower ragged robin (Hauser ally, as in many insect, plant and mouse studies. This is partly & Loeschke, 1996) and Drosophila (Dahlgaard & Ho€mann, because any e€ects will be stronger and hence more detectable 2000). Field evidence for inbreeding depression and its in extreme cases and partly because of the diculties of interaction with stress comes from experimental work on quantifying the level of inbreeding where the relatedness of white-footed mice (Jimenez et al., 1994). When wild-caught parents is too distant to be determined by pedigrees. An mice are subject to full-sib mating in captivity for three of four unwitting consequence is the perception that inbreeding e€ects generations, and then returned to the capture site, they show occur almost exclusively in `incestuous' families where the greater mass loss and lower survivorship than the progeny of parents share very recent common ancestry. However, new outbred matings. When equivalent mice are retained in the techniques now allow any level of inbreeding to be determined laboratory, inbreeding depression is much less severe. with ease and without the need for pedigrees. Essentially more There is now also growing evidence that inbreeding depres- sophisticated variants of crude heterozygosity, these measures sion can impact not just individuals but entire populations. assess parental similarity via the similarity of paternally and For instance, the Mandarte Island song sparrow persists close maternally inherited alleles. For example, the squared di€er- to the edge of viability at a peak size of some 200 birds (Keller ence in length between microsatellite alleles provides a measure et al., 1994; Keller, 1998). Occasional hard winters result in that is expected to scale linearly with time since common population crashes in which only a handful of birds survive. ancestor. Calculated between alleles at a locus and then Pedigree analysis shows that relatively inbred individuals are averaged over many loci, the resulting statistic, named mean more likely to die in crashes than those whose parents were less d2, re¯ects the overall genomic similarity of an individual's related. A similar e€ect is seen on a larger scale in Glanville parents (Coulson et al., 1998). fritillary butter¯ies in Finland, where many, widely dispersed Using mean d2 and related measures such as standardized and sometimes very small patches of food plant provide heterozygosity (Coltman et al., 1999), a measure of average heterozygosity which allows for di€erences in variability among loci, the consequences of parental similarity are now being extended greatly to embrace all individuals, not just `incestuous' families. In deer and seals, mean d2 predicts juvenile survival, the progeny of genetically dissimilar parents being heavier and surviving better (Coltman et al., 1998; Coulson et al., 1998). In sheep, standardized heterozygosity explains signi®cant variation in parasitic worm burdens, which in turn in¯uence survival (Coltman et al., 1999). Subtle e€ects can even be detected in humans. Using data from a whole genome screen for genetic factors conferring susceptibility to tuberculosis, disease incidence was associated with low mean d-squared values on chromosome 15, coincident with a region identi®ed by traditional linkage mapping as contributing to disease susceptibility (Bellamy et al., 2000). Just as with the sheep, the probability of infection appears to be increased by recessive factors. Parental similarity can also modulate adult reproductive success. Inbred mice released into seminatural populations show lower success compared with outbred controls (Meagher et al., 2000), and molecular measures of parental dissimilarity correlate positively with reproductive success in a range of vertebrates including deer (Slate et al., 2000) and seals, Fig. 3 Hatching success of six one-pair lines of Bicyclus albatrosses and pilot whales (Amos et al., in press). If these anynana, compared with that of outbred control lines. All ®ndings prove to be the rule rather than the exception, there lines show some recovery from initial inbreeding depression, are important consequences for mating behaviour and for the and three lines show a marked recovery. Redrawn from maintenance of genetic diversity. Saccheri et al., 1996.

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In most systems, sexual selection favours choice for high- been purged under one environmental challenge, retained quality partners who have inherited desirable qualities from enhanced susceptibility to unrelated challenges, suggesting that their parents. This reduces variability because reproduction is purging may not be general, but instead may involve a (small?) skewed towards a small number of dominant lineages. In subset of all genes which contribute to inbreeding depression. contrast, selection for dissimilar parental combinations In a second series of experiment, Reed & Bryant (2000) studied increases variability. Which of these opposing forces will how inbreeding depression varied over time for populations of `win' will undoubtedly vary from species to species, but early di€erent sizes, including one where a population was forced results indicate that preference for parental dissimilarity may through an e€ective population size of 5 for one generation be widespread. Thus, male guppies both recognize and prefer (`founder ¯ush'). Their results show that, although some unfamiliar females (Kelley et al., 1999), while in grey seals, recovery is seen in the founder ¯ush populations, in general, females who mate with di€erent males in di€erent seasons give the inbreeding e€ects persist for at least 24 generations, rather birth to pups who are less paternally related to one another longer than implied by the Bicyclus experiments. Finally, and than expected by chance, suggesting females prefer diverse perhaps most signi®cantly, thorough reviews covering 25 mates (Amos et al., 2001). Preferences of this nature will tend populations of mammals (Ballou, 1997) and 53 plant studies to preserve variability and may provide small and declining (Byers & Waller, 1999) conclude that purging reduces inbreed- populations with an unexpected extra resilience against both ing depression only in some traits, only in some populations, inbreeding depression and genetic erosion. and even then, only to a rather limited extent (Fig. 4). An alternative answer to the problem of inbreeding depres- sion is based on introduction of new genetic diversity from Addressing inbreeding depression in species other populations. In the Mandarte Island sparrow study it conservation was noted that, following each crash, genetic diversity was Inbreeding depression can potentially contribute to a so-called considerably reduced, but that levels rapidly recovered as the (Lacy & Lindenmayer, 1995), in which population expanded again (Keller et al., 1994; Keller, 1998). decline reduces ®tness which in turn hastens the decline, Because regeneration of variability by would take increasing both inbreeding depression and vulnerability to vastly more time, the explanation lay with the one or two stochastic events in a destructive feedback loop. Two strategies immigrants that enter the population each year. These new that have been proposed to counteract this phenomenon raise individuals bring in new diversity and, for the ®rst generation a number of complicated and unsolved issues. or so, would enjoy excellent reproductive success by dint of One idea is to adopt a short-term policy of forced their being unrelated to any members of the remnant popu- inbreeding in order to expose deleterious recessive alleles and lation they joined. Likewise, deliberate introduction of indi- thereby purge them from the population. Some species that viduals from elsewhere has been proposed as a management have been through documented bottlenecks have subsequently strategy for several isolated and extremely small populations shown dramatic recoveries. Examples include the northern thought to be su€ering inbreeding depression (e.g. (Hedrick, elephant seal, Antarctic fur seals, the southern right whale, the 1995; Hedrick et al., 1997), and, in the case of the Mexican southern white rhino and the Mauritius kestrel. These striking wolf, has already been carried out (Kalinowski et al., 1999). rebounds could be explained by a reduction in intraspeci®c Nevertheless, this approach raises several potential prob- competition, or by the loss of coadapted parasites and lems (Lynch, 1996). Of general concern is the question of diseases. However, strong purging during the nadir of each hybridization. There is a ®ne dividing line between on the one species' bottleneck could also have increased ®tness by hand the contamination of indigenous gene pools by intro- decreasing the genetic load. More direct evidence of purging duced or escaped exotic species, a problem which many hold to comes from experiments like that on Bicyclus (in Fig. 3), where be a major threat (Crivelli, 1995; Levin et al., 1996; Rhymer & fecundity in highly inbred lines can recover to normal levels Simberlo€, 1996; Huxel, 1999), and, on the other, the after only a few generations, despite continued inbreeding deliberate augmentation of an endangered population using (Saccheri et al., 1996). a genetically distinct lineage from elsewhere. Is it vital to However, in the Bicyclus example, recovery is not guaran- preserve only biodiversity in its current state (naõÈ vely con- teed, and around half the lines show little rebound. Supporting sidered by some to be pristine), or can we assume that natural this observation, Hedrick (1994) provides models showing how selection will successfully preserve locally important adapta- the consequences of full-sib mating may depend on the genetic tions while at the same time assimilating the best of what has basis of inbreeding depression. Where this is the consequence been introduced (Rhymer & Simberlo€, 1996; see Patten, 2000 of lethals, purging can occur without substantially increasing for a recent case study)? The jury is still out. extinction, but when inbreeding depression is the result of More pragmatically, augmentation of a very small popula- many detrimental alleles each of small e€ect, forced inbreeding tion with individuals from other, larger populations may also can cause ®xation rather than purging, leading to a reduction refresh the local pool of detrimental recessives. These new in overall ®tness. Recent experimental work on ¯ies suggests recessives would increase in frequency in parallel with the that not only is the expression of inbreeding depression itself success enjoyed by their bearers. If the receiving population stress-dependent, but also that the relationship between has already survived a severe bottleneck, it may have been population history and purging may be complex. First, purged of many of the strongest factors contributing to its Dahlgaard & Ho€mann (2000) showed that ¯ies that had genetic load. Consequently, the introduction of new detrimental

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Regardless of whether introduced individuals bring with them intrinsically deleterious mutations, i.e. those which will cause loss of ®tness in any genetic background, there is also the potential problem of outbreeding depression (Lynch, 1996). will tend to create populations that are adapted to the local conditions. Augmentation introduces genetic material that is adapted to a di€erent set of conditions. In some cases, the result is disastrous. For example, when a population of Tatra mountain ibex in Czechoslovakia was `enriched' by new animals from Sinai and Turkey, the o€spring inherited an inappropriate calving date, giving birth in mid- winter and losing their progeny (Greig, 1979). Other similar threats include the breaking up of coadapted gene complexes and the introduction of new diseases to which local popula- tions may lack resistance. It seems likely that most augmen- tation exercises will, at some level, involve risks of this kind. The question is, whether the bene®ts brought by reduced inbreeding and an increase in numbers will outweigh the tangible threats of outbreeding depression and the need to purge any new deleterious recessives carried by the new blood. As yet, there are no ®rm rules. One important issue may be the size of the two populations under consideration. Where the donor population has itself been small for some time (as in the case of all Mexican wolf lineages; see Hedrick et al., 1997), it may carry a low genetic load, and hence the risk of its individuals introducing deleteri- ous alleles into the recipient population may be slight. How- ever, where the donor population is large, it is less likely to have been purged of its genetic load, and hence direct introduction of its members into a far smaller population may be ill-advised. One way around this could be deliberate inbreeding of individuals from the large donor population, to purge them of deleterious alleles prior to any introduction, though this may Fig. 4 Captive neonatal survival in 19 mammal species for not be a cost-e€ective approach to saving a small population of noninbred animals, inbred animals (inbred 1), and inbred a species which is more abundant elsewhere. animals with inbred ancestors (inbred 2). There is signi®cant inbreeding depression in seven species (with asterisks indica- ting *P < 0.05, **P < 0.001, ***P < 0.001). Although 15 Mutational meltdown species show a trend towards higher survival for inbred One ®nal genetic problem which may threaten small popula- animals with inbred ancestors than for other inbred animals, tions is mutational meltdown (Lynch, 1993). The idea is that this purging e€ect is signi®cant only for Sumatran tigers. deleterious mutations need to be removed by natural selection, Redrawn from Ballou, 1997. ESHREW, elephant shrew; GLT, and this usually invokes di€erential mortality among individ- golden lion tamarin (***); GOELDI, Goeldi's marmoset (***); uals carrying greater or lesser genetic loads. If a population GGAL, greater galago; KERODON, kerodon; BORIS, boris; starts declining, the number of individuals available to get rid MWOLF, Mexican wolf; RPANDA, red panda; ALION, of deleterious mutations by selective death dwindles, purging Asiatic lion; STIGER, Sumatran tiger; PHORSE, Przewalski's becomes less and less ecient, the load will build up and the horse; PHIPPO, pygmy hippopotamus (***); MUNTJAC, average ®tness of the population will fall. The result could be a muntjac; EDEER, Eld's deer (***); GAUR, gaur; BISON, form of destructive feedback loop similar to the extinction European bison (**); DORCAS, Dorcas gazelle (*); SPEKES, vortex. Theoretical studies suggest the process will be most Speke's gazelle (***); THAN, nilgri tahr. important in small asexual populations (Lynch, 1993) although it cannot be ruled out even in large sexual popula- tions (Bernardes, 1995). alleles could increase its vulnerability to any future bottleneck. Despite the theoretical precedent, an experimental analysis In the case of the Mandarte sparrows, it is intriguing to of genetic load in fruit ¯ies ± assessed as the di€erence in speculate that the repeated population crashes are actually ®tness between individuals removed from a population and facilitated by incoming `new blood'. Hence, although genetic inbred, or removed and not inbred ± reveals that even after augmentation may result in a short-term increase in ®tness, 45 generations, load does not di€er between populations with this may be followed by a subsequent decline. Ne ˆ 25, and populations with Ne set to 500 (Fig. 5; see

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slowly and `useful' variability is likely to be lost more slowly still. Recent studies have `muddied the waters' by showing that inbreeding e€ects are not restricted to reduced fecundity in matings between close relatives. Instead, dissimilar pairs may produce ®tter than average o€spring, and breeding behaviour often seems to have evolved to exploit this fact. Such patterns could have a profound in¯uence on the genetic dynamics of threatened populations, and suggest that theoretical models based entirely on random mating need to be revisited. At the same time, the importance of diversity in resisting pathogens is becoming more apparent, and the ®rst studies measuring evolutionary potential have shown that it is reduced in inbred lines of fruit ¯ies. The key question is whether these trends re¯ect what actually happens in nature, or whether most species have evolved suites of behaviours that negate the worst Fig. 5 The genetic load of fruit¯y populations maintained for aspects of inbreeding depression and allow unexpectedly ecient retention of useful variability. With many highly 45 generations at Nes of 25, 50, 100, 250 and 500, and of base and wild control populations. Genetic load is assessed as the detailed studies in the pipeline, we are optimistic that answers di€erence in ®tness of noninbred and inbred lines, measured will soon emerge, and that the current patchwork of often- under competitive and benign conditions, and expressed as contradictory observations can be woven into a more uni®ed load relative to the base population. Bars give means and explanatory framework. The more complete this becomes, the standard errors. (Redrawn from Gilligan et al., 1997.) more e€ective management strategies will be at countering genetic threats constructively and with minimal risk. Gilligan et al., 1997). These results suggest that, even though mutational meltdown may pose a problem for populations Acknowledgements held at low levels for very long periods of times, it is unlikely to We are very grateful to Tim Caro and one anonymous a€ect small populations over the 100-year timescales con- reviewer for perceptive comments that helped to improve this sidered by most conservation planners. The reason for this manuscript considerably. discrepancy between theoretical predictions and experimental systems is unclear. Drosophila may be the exception rather than the rule. Perhaps more likely is the possibility that cryptic References mate choice, sperm competition, di€erential reproductive rates , W.AND HARWOOD , J. 1998. 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